Palbociclib combined with endocrine therapy is approved for treating patients with hormone-receptor-positive/human epidermal growth factor receptor 2-negative (HR+/HER2-) advanced breast cancer; however, data on palbociclib combined with tamoxifen are limited. We investigated the efficacy and safety of palbociclib-tamoxifen in patients with HR+/HER2- advanced breast cancer. This double-blind phase 3 study included 184 women who were randomly assigned 1:1 to receive palbociclib-tamoxifen or placebo-tamoxifen. Pre/perimenopausal women also received goserelin. The primary endpoint was investigator-assessed progression-free survival (PFS). Secondary endpoints included overall survival (OS) and safety. Median PFS was 24.4 months (95% confidence interval [CI], 13.1-32.4) with palbociclib-tamoxifen and 11.1 months (95% CI, 7.4-14.6) with placebo-tamoxifen (hazard ratio [HR], 0.60; 95% CI, 0.43-0.85; P = 0.002). Palbociclib-tamoxifen improved PFS in patients who were treated with first-line or second-line endocrine therapy and pre-, peri-, and postmenopausal patients. Though OS data are still immature (median not reached in both groups), an overall risk reduction of 27% (HR, 0.73; 95% CI, 0.44-1.21) with palbociclib-tamoxifen was observed at the time of PFS analysis. The most common grade 3/4 adverse event with palbociclib-tamoxifen was neutropenia (89.0% [none were febrile] versus 1.1% with placebo-tamoxifen). There were no deaths owing to adverse events in either group. Among patients with HR+/HER2- advanced breast cancer, palbociclib-tamoxifen resulted in significantly longer PFS than tamoxifen alone. Early OS data showed a trend favoring palbociclib-tamoxifen. Trial registration: ClinicalTrials.gov number, NCT03423199. Study registration date: February 06, 2018.